In breech-loading artillery guns, for example, where the gun is loaded by first ramming an explosive shell or other projectile through the breech and into the gun tube or barrel to a normal firing position and then inserting a separate propelling charge behind projectile, with a free space therebetween, friction between the projectile rotating band and barrel rifling is relied on to maintain the projectile in its firing position. When the gun is elevated, either before or after the propelling charge is inserted the projectile can unseat and fall back from its firing position, with one of two results when the gun is fired: either the propellant gases blow past the projectile leaving the projectile in the gun chamber; or the projectile jams in the barrel, causing a catastrophic explosion which can destroy the gun and its mount and kill or maim the gun crew. Projectile fallback can occur in practically any gun of 155mm size, or larger. For example, a 155mm M107 explosive shell for an M185 Howitzer Cannor weighs about 97 pounds and has a length of 27 to 28 inches.
An object of the present invention is to provide means for effectively limiting the fallback of a projectile from its firing position to a harmless amount. In accordance with the invention, the gun barrel is formed with at least one forwardly-facing locking shoulder located near the rear end of the projectile in its firing position, and the projectile is provided with at least one outwardly-movable detent or other means having a rearwardly-facing locking shoulder located near the barrel shoulder, and means for automatically moving the detent into the path of the barrel shoulder, on fallback movement of the projectile from its firing position to a locking position determined by the two locking shoulders. Preferably, the barrel shoulder is provided by an internal annular recess in the forcing cone of the barrel, and the projectile detent is a rearwardly-and-outwardly-extending, resilient, flexible strip integral with a plastic obturator ring or band attached to the projectile just behind the usual rotating band thereon. On the other hand, the projectile detent may be an outwardly-movable, spring-pressed detent plunger slidably mounted in an outwardly-open recess in the projectile.